Prior to removal of a brain tumor from under the cranial bone, the cranial bone has to be opened to provide access to the tumor. To this end, after the introduction of at least one burr hole into the cranial bone, a bone flap is sawed out of the cranial bone. After surgical removal of the tumor, the cranial bone has to be closed again. To this end, the previously sawed-out bone flap is generally refixed to the cranial bone by means of suitable implants. These implants are frequently also used to cover the burr holes.
Various implants are known for fixing a bone flap to a cranial bone and for covering burr holes. For instance, a brochure by Codman & Shurtleff from 1965 describes the “Todd Burr Hole Button.” This implant comprises a first, disc-shaped bearing part with a shank projecting centrally from this bearing part. A second, likewise disc-shaped bearing part has a central opening and is threaded onto the shank in such a way that the latter extends through the opening.
Like the two bearing parts, the shank is made from a resilient plastics material and has an external diameter which is somewhat larger than the internal diameter of the opening in the second bearing part. A tensile force acting axially on the shank therefore brings about resilient extension of the shank and a reduction in the external shank diameter associated with the extension. As a consequence of this reduction in external diameter, the second bearing part may be threaded onto the shank and moved along the shank towards the first bearing part. If the tensile force acting on the shank is reduced, the external diameter of the shank increases again and ensures tension-proof coupling of the second bearing part with the shank and thus also with the first bearing part provided at the shank end.
A number of such implants are used to fix a bone flap to a cranial bone. Generally, a shank acting as coupling element for the two bearing parts is inserted into a gap between the bone flap and the cranial bone in such a way that the bone flap and the cranial bone are clamped between the two bearing parts.
The implant known from the Codman & Shurtleff brochure has not become established in practice. One of the reasons for this is the difficulty in handling the implant. For instance, the application of a tensile force to the shank and the displacement of a bearing part along the shank while the tensile force is maintained has proved extremely awkward. In addition the implant is not capable of maintaining high clamping forces reliably and in the long term.
JP 05 220 174 A discloses a further implant for fixing a bone flap to a cranial bone. The implant comprises a rectangular first bearing part with a strap projecting there-from and a rectangular second bearing part. The second bearing part has a central opening and is threaded onto the strap in such a way that the strap extends through the opening.
The implant strap is provided on one or both sides with teeth. The bearing part threaded onto the strap has up to two movable claws, that engage with the teeth provided on the strap. Both the teeth and the claws have a sawtooth-like profile, that provides less resistance to movement of the threaded-on bearing part towards the other bearing part than to movement in the opposite direction. This situation ensures tension-proof coupling of the two bearing parts, in such a way that the bone flap and the cranial bone may be clamped reliably therebetween.